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Tekkaya, Ahmet Eeman
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Tekkaya, A. E.
Tekkaya, A. Erman
Tekkaya, AE
A.E.Tekkaya
Tekkaya,A.E.
T.,Ahmet Eeman
A.,Tekkaya
A., Tekkaya
T., Ahmet Eeman
Tekkaya, Ahmet Eeman
Ahmet Eeman, Tekkaya
Tekkaya, E
Tekkaya, E. A.
Tekkaya, A. Erman
Tekkaya, AE
A.E.Tekkaya
Tekkaya,A.E.
T.,Ahmet Eeman
A.,Tekkaya
A., Tekkaya
T., Ahmet Eeman
Tekkaya, Ahmet Eeman
Ahmet Eeman, Tekkaya
Tekkaya, E
Tekkaya, E. A.
Job Title
Profesör Doktor
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Main Affiliation
Manufacturing Engineering
Status
Former Staff
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Sustainable Development Goals
9
INDUSTRY, INNOVATION AND INFRASTRUCTURE
4
Research Products
Scholarly Output
31
Articles
14
Citation Count
867
Supervised Theses
2
31 results
Scholarly Output Search Results
Now showing 1 - 10 of 31
Article Citation - WoS: 15Finite deformation plasticity coupled with isotropic damage: Formulation in principal axes and applications(Elsevier Science Bv, 2010) Soyarslan, C.; Tekkaya, A. E.; Manufacturing Engineering; Department of Mechatronics EngineeringA local, isotropic damage coupled hyperelastic-plastic framework is formulated in principal axes. It is shown that, in a functional setting, treatment of many damage growth models, including those originated from phenomenological models (with formal thermodynamical derivations), micromechanics or fracture criteria, proposed in the literature, is possible. As a model problem, a Lemaitre-variant damage model with quasi-unilateral damage evolutionary forms is given with special emphasis on the feasibility of formulations in principal axes. To this end, closed form expression for the inelastic tangent moduli, consistent with the linearization of the closest point projection algorithm, is derived. It is shown that, generally, even in the absence of quasi-unilateral damage evolutionary conditions, the consistent tangent moduli are unsymmetric. The model is implemented as a user defined material subroutine (UMAT) for ABAQUS/Standard. The predictive capability of the selected model problem is studied through axi-symmetric application problems involving forward extrusion of a cylindrical billet, upsetting of a tapered specimen and tension of a notched specimen, in which characteristic failure mechanisms are observed. (C) 2010 Elsevier B.V. All rights reserved.Conference Object Analysis of Force-Indentation Measurements on Anisotropic Metals(Edizioni libreria progetto, 2005) Koray,E.; Topcu,N.; Tekkaya,A.E.; Manufacturing EngineeringIndentation tests are widely used with simultaneous measurements of indentation depth and force especially for determining material properties. In this study, findings on parameters affecting the force-displacement curves obtained from indentations on anisotropic metals using special not self-similar indenters will be presented. Parameters such as specimen dimensions, friction, surface roughness, specimen clamping and indentation spacing have been investigated by finite element analysis and by experiments. Optimum values of these parameters to achieve higher repeatability and accuracy are determined.Article Citation - WoS: 5Citation - Scopus: 6Material Flow Control in High Pressure Sheet Metal Forming of Large Area Parts With Complex Geometry Details(verlag Stahleisen Mbh, 2005) Trompeter, M; Önder, E; Homberg, W; Tekkaya, E; Kleiner, M; Manufacturing EngineeringWorking media based forming processes show advantages compared to the conventional deep drawing in the range of sheet metal parts with complex geometry details. By High Pressure Sheet Metal Forming (HBU), complex parts can be formed with reduced tool costs, fewer process steps, and improved part properties, particularly by the use of high strength steels. In order to use these advantages to full capacity, the material flow into the area of the geometry details needs to be optimised. The key element for the material flow control is a multi-point blank holder. In combination with flange draw-in sensors, a closed loop flange draw-in control can be built up which guarantees a reproducible material flow and, consequently, defined part properties. Furthermore, a favourable pre-distribution of sheet metal material can be reached which leads to a widening of the process limits. Considering a large area sheet metal part with a complex door handle element as example, strategies for the material flow control will be discussed in this paper. The conclusions are based on FE-simulations as well as experimental findings.Conference Object Citation - Scopus: 2Influence of Heat Treatment and Grinding Conditions on Surface Residual Stresses in the Production of Rollers(Trans Tech Publications Ltd, 2006) Güley,V.; Tekkaya,A.E.; Savaş,T.; Özhan,F.; Manufacturing EngineeringThe aim of this study is to investigate surface residual stresses after heat treatment and grinding processes in the production of rollers. The residual stresses were measured using the X-ray diffraction method utilizing chromium radiation, which has an average penetration depth of 5 μm incident on AISI-E52100 (100Cr6) ball bearing steel. Taguchi design of experiments (DOE) is applied to define the set of experiments for grinding, which facilitates evaluation of the individual influences of process parameters on residual stresses and also eliminates unnecessary experiments. Response of residual stresses to each parameter is evaluated with the help of the results of residual stress measurements by X-ray diffraction. In grinding with aluminum oxide wheels, it was concluded that the lower the cutting speed and the higher the workpiece speed the higher the magnitude of surface compressive residual stresses. Higher compressive stresses were measured in axial direction compared to the circumferential direction after the grinding process.Conference Object Citation - WoS: 1Experimental Investigation of Residual Stresses After Heat Treatment and Grinding Processes in the Production of Ball Bearing Rings(Trans Tech Publications Ltd, 2008) Gueley, Volkan; Tekkaya, A. Erman; Savas, Turhan; Ozhan, Feridun; Manufacturing EngineeringExperimental investigation of residual stresses after heat treatment and grinding processes in the production of ball bearing rings has been carried out. The residual stresses were measured by X-ray diffraction method utilizing chromium radiation, which has an average penetration depth of 5 mu m incident on 100Cr6 (AISI-E52100) ball bearing steel. The process parameters of heat treatment and grinding processes were varied so as to represent the extreme values that can be applied in the respective processes. Hardness and percent retained austenite limit the heat treatment process parameters; while roundness, surface roughness and form the grinding process. Tensile surface residual stresses on the raceway of ball bearing rings changes to compression after grinding in both circumferential and axial directions. In grinding relatively higher compressive stresses were measured in axial direction compared to the circumferential direction. This experimental investigation also showed that the influence of heat treatment process parameters on the magnitude and distribution of residual stresses survived even after grinding process; i.e. heat treatment and grinding processes cannot be evaluated independently in process design for favourable residual stresses.Article Citation - WoS: 60Citation - Scopus: 68A Semi-Empirical Approach for Residual Stresses in Electric Discharge Machining (edm)(Elsevier Sci Ltd, 2006) Ekmekci, Bulent; Tekkaya, A. Erman; Erden, Abdulkadir; Department of Mechatronics Engineering; Manufacturing EngineeringHigh residual stresses are developed on the surfaces of electric discharge machined parts. In this study, layer removal method is used to measure the residual stress profile as a function of depth beneath the surface caused by die sinking type EDM. Cracking and its consequences on residual stresses are also studied on samples machined at long pulse durations. A modified empirical equation is developed for scaling residual stresses in machined surfaces with respect to operating conditions. In this model, a unit amplitude shape function representing change in curvature with respect to removal depth is proposed. The proposed form is found to be a special form of a Gauss Distribution. It is the sum of two Gaussian peaks, with the same amplitude and pulse width but opposite center location. The form can be represented by three constant coefficients. These coefficients depend on the released energy by a power function. (C) 2005 Elsevier Ltd. All rights reserved.Conference Object Citation - Scopus: 3Experimental Investigation of Residual Stresses After Heat Treatment and Grinding Processes in the Production of Ball Bearing Rings(Trans Tech Publications Ltd, 2008) Güley,V.; Tekkaya,A.E.; Savaş,T.; Özhan,F.; Manufacturing EngineeringExperimental investigation of residual stresses after heat treatment and grinding processes in the production of ball bearing rings has been carried out. The residual stresses were measured by X-ray diffraction method utilizing chromium radiation, which has an average penetration depth of 5 μm incident on 100Cr6 (AISI-E52100) ball bearing steel. The process parameters of heat treatment and grinding processes were varied so as to represent the extreme values that can be applied in the respective processes. Hardness and percent retained austenite limit the heat treatment process parameters; while roundness, surface roughness and form the grinding process. Tensile surface residual stresses on the raceway of ball bearing rings changes to compression after grinding in both circumferential and axial directions. In grinding relatively higher compressive stresses were measured in axial direction compared to the circumferential direction. This experimental investigation also showed that the influence of heat treatment process parameters on the magnitude and distribution of residual stresses survived even after grinding process; i.e. heat treatment and grinding processes cannot be evaluated independently in process design for favourable residual stresses.Article Citation - WoS: 9Citation - Scopus: 13Free forming of locally heated specimens(Elsevier Sci Ltd, 2007) Okman, O.; Ozmen, M.; Huwiler, H.; Tekkaya, A. E.; Manufacturing EngineeringA novel manufacturing method is investigated, in which a steep temperature gradient within the workpiece is induced to facilitate material flow locally. By this method, complex shapes can be formed without complicated dies. The feasibility of the idea is analyzed experimentally and numerically. Local heating is realized either by means of induction or laser heating. Experiments using materials 16MnCr5, X5CrNi18/9, and Ti6Al4V have been conducted under various process conditions. These experiments have also been modeled by finite element method (FEM) validating the analysis procedure. Electromagnetic models are used to analyze the heat generation pattern on the workpiece by induction. It is found that the most important process parameters are the thermal diffusivity and the temperature sensitivity of the flow curve of the workpiece material. The lower the thermal diffusivity and the higher the temperature sensitivity, the more differentiated local shapes can be formed. For the analyzed geometries, induction heating has been observed to be more effective. Deformation rate and initial workpiece geometry have also a significant effect on the achievable local deformations. Various failure modes such as unintended deformations, damage by fracture, and melting of the workpiece material are described. It is concluded that the new idea of forming local shapes by local heating is a feasible and controllable manufacturing alternative. (c) 2006 Elsevier Ltd. All rights reserved.Review Citation - WoS: 55Citation - Scopus: 58The Development of Ring Rolling Technology - Part 2: Investigation of Process Behaviour and Production Equipment(Wiley-blackwell, 2005) Allwood, JM; Tekkaya, AE; Stanistreet, TF; Manufacturing EngineeringThis is the continuation of a thorough survey of work on ring rolling published in the English and German languages up to 2004 [see No. 2/3, p. 111-120]: The methods used to investigate the process are reviewed, separated into experimental and theoretical categories; the insights gained from these investigations are organised according to the challenges identified at the outset; developments in the control and operation of the process are described. Having given a set of ideal targets for the process, the state of current knowledge about ring rolling is assessed in order to predict likely developments: process modelling capability is nearly able to predict rolling behaviour for a complete cycle with sufficient accuracy to allow effective use of models for design of rolling schedules and preforms; analysis of material behaviour is relatively mature for steel rings, but has scope for significant extension for titanium and aluminium alloys and composites; design choices that seek to extend the flexibility of the process have had some exploration, but could be extended.Article Citation - WoS: 38Citation - Scopus: 42Application of Continuum Damage Mechanics in Discontinuous Crack Formation: Forward Extrusion Chevron(Wiley-v C H verlag Gmbh, 2008) Soyarslan, Celal; Tekkaya, A. Erman; Akyuz, Ugurhan; Department of Mechatronics Engineering; Manufacturing EngineeringMaterializing Continuum Damage Mechanics (CDM), numerical modeling of discrete internal cracks, namely central bursts, in direct forward extrusion process is presented. Accordingly, in a thermodynamically consistent setting, a local Lemaitre variant damage model with quasi-unilateral evolution is coupled with hyperelastic-plasticity. The formulations are constructed in the principal axes where simultaneous local integration schemes are efficiently developed. To this end, the framework is implemented as ABAQUS/VUMAT subroutine to be used in an explicit FE solution scheme, and utilized in direct forward extrusion simulations for bearing steel, 100Cr6. Discontinuous cracks are obtained with the element deletion procedure, where the elements reaching the critical damage value are removed from the mesh. The periodicity of the cracks shows well accordance with the experimental facts. The investigations reveal that, application of the quasi-unilateral conditions together with the crack closure parameter has an indispensable effect on the damage accumulation zones by determining their internal or superficial character. (C) 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
